The internal intensity modulation and radiation pulse generation in high-power CO.sub.2 lasers, which take place directly in the laser resonator, is indispensible for extending the possible uses of laser techniques, especially within the setting of the aforementioned areas of application. In particular, for such a modulation and generation of radiation pulses, possibilities are required for external triggering in the sense of freely selectable parameters especially for the pulse peak power or pulse duration and the pulse repetition rate up to aperiodic pulse repetition, a simultaneous wavelength selection being desirable or absolutely essential in numerous applications.
The principle of the transversely excited high-pressure gas laser (TEA laser), which permits exceptionally high peak pulse powers to be realized because of the high pressures of the laser gas, offers one possibility of generating gas laser pulses. However, achieving the high pulse repetition rates is exceptionally problematical in the case of this principle. For example, frequencies of a few hundred hertz already require an enormous technical effort.
The different methods of Q-switching of laser resonators lead to high magnifications of the peak pulse power relative to the power in continuous operation. All previously realized variations, however, have significant disadvantages with respect to laser applications in the areas of use listed above.
A method of gas-laser pulse generation disclosed by Flynn et al. (IEEE Journal of Quantum Electronics, vol. QE-2, 378 (1966)), the active Q-switching by means of a rotating mirror, is unsuitable especially for material processing, since it results in an unfavorable, time-related duty ratio of laser on to laser off and thus in an appreciable loss of average radiation power. Moreover, only periodic pulse repetitions can be achieved.
In Applied Physics Letters 11, 88 (1967), the method of passive Q-switching of CO.sub.2 lasers by means of special absorbing gases, which are disposed in the resonator, is described. The disadvantage here is that, due to the gases used and as a function of the respective gas mixture, only a particular pulse repetition rate, which is of the order of a few kHz, is allowed and, on the other hand, the average, switchable laser power is also limited.
The method of modulating by means of the electrooptical effect in crystals, which is very effective in the visible range of the spectrum, is realizable only at great expense in the middle range of the infrared around a wavelength of 10 micrometers, since it requires in this range large crystal lengths which, on the one hand, are associated with unavoidable, relative high absorption losses and, on the other, with high costs, and high control voltages for sufficient depths of modulation (in this connection, see, for example, IEEE Journal of Quantum Electronics, vol. QE-2, 243 (1966)).
The intensity of the laser can also be controlled by using an interferometer arrangement with a selective transmission instead of the partially transparent uncoupling mirror. Such an arrangement is described in German Offlegungschrift No. 2,223,945 as well as in German Offenlegungschrift No. 2,044,280. In both Offenlegungsschriften, electrooptical or magnetooptical effects are proposed for tuning the interferometer arrangement. The two arrangements described are typical, firstly of lasers in the visible range of the spectrum and secondly of lasers of relatively low average output power. Their use in high-power CO.sub.2 lasers does not make possible the desired effect of the wavelegth-selective intensity modulation and radiation pulse generation.
A further method, the Bragg diffraction at acoustooptically generated phase gratings suffers from the limited diffraction efficiency, which leads to losses and a relative shallow depth of modulation, which is unsuitable for processing materials with CO.sub.2 lasers.
Technically, the possibility of modulating the discharge current of the laser gas discharge is used predominantly. Basically however, this mechanism limits the maximum achievable modulation frequency, if the requirement of a sufficient depth of modulation is to be fulfilled. The limiting frequency for high-power CO.sub.2 lasers is at about 2.5 kHz, increasing losses in laser power occurring as the frequency increases above about 1 kHz. Moreover, the mechanism of electrically pulsing the gas discharge limits the achievable magnification of the peak pulse power to a factor of 10, since the processes of the build-up of the population inversion by the gas discharge and the decay of the inversion by the oscillation build-up of the laser overlap in time.
The arrangement proposed in German Offenlegungsschrift No. 2,816,659 represents an attempt to increase the limiting frequency. In this arrangement, the gas laser has at least two gas discharge tubes, connected in series and which can be pumped consecutively with a defined time difference by means of appropriate current pulses. By these means, it is possible to increase the limiting frequency, specified by the pumping mechanism of the gas laser. However, the energy of the individual radiation pulses falls corresponding to the increase in frequency. Moreover, the cost of the equipment increases significantly.